Lesions in various brain areas increased locomotor activity in rats. These areas constitute a functionally inter-related system which is proposed as a "hyperkinetic pathway". The major areas under study are the frontal lobe, thalamus (dorsomedial nucleus and intralaminar nuclei), substantia nigra and mesencephalic reticular formation. Increased locomotor behavior or hyperactivity in rats cannot be regarded as a simple alteration of behavior. An analysis of the behavior of rats, which have been made hyperactive by injections of amphetamine, shows that sequences of behavioral acts are altered when the rats are hyperactive. In addition, these hyperactive rats shorten the duration of performance of many behavioral acts and also increase the frequency of occurrence of many acts. Thus three kinds of behavioral changes are involved in amphetamine-induced hyperactivity. The nature of hyperactivity caused by brain lesions is being analyzed in the same way to determine if different lesions produce different kinds of hyperactivity. It seems probable, based on preliminary data, that not all lesions will cause the same changes as those produced by amphetamine.